The present invention relates to an outermost surface on steel, particularly stainless steel having a high chromium content. The present invention provides an outermost surface on steels which surface provides enhanced materials protection (e.g. protects the substrate or matrix). The surface reduces coking in applications where the steel is exposed to a hydrocarbon environment at high temperatures. Such stainless steel may be used in a number of applications, particularly in the processing of hydrocarbons and in particular in pyrolysis processes such as the dehydrogenation of alkanes to olefins (e.g. ethane to ethylene); reactor tubes for cracking hydrocarbons; or reactor tubes for steam cracking or reforming.
It has been known for some time that the surface composition of a metal alloy may have a significant impact on its utility. It has been known to treat steel to produce an iron oxide layer that is easily removed. It has also been known to treat steel to enhance its wear resistance. The use of stainless steels has heretofore relied upon the protection (e.g. against corrosion and other forms of material degradation) afforded by a chromia surface. As far as Applicants are aware there is not a significant amount of art on treating steels to significantly reduce coking in hydrocarbon processing. There is even less art on the types of surface that reduce coking significantly in hydrocarbon processing.
There has been experimental work related to the nuclear industry that spinels similar to the present invention can be generated on stainless surfaces. However, these spinels are thermo-mechanically unstable and tend to delaminate. This is a limitation which tends to teach against using such surfaces commercially. These surfaces have been evaluated for use in the nuclear industry but to Applicants"" knowledge have never been commercially used.
In the petrochemical industry due to its thermo-mechanical limitations spinels similar to the present invention are believed to be overall less protective than chromia. It is also believed from a coke make perspective spinels similar to the present invention are not considered to be more catalytically inert than chromia. Due to these teachings, to Applicants"" knowledge, such spinels have not been produced for use in the petrochemical industry.
U.S. Pat. No. 3,864,093 issued Feb. 4, 1975 to Wolfla (assigned to Union Carbide Corporation) teaches applying a coating of various metal oxides to a steel substrate. The oxides are incorporated into a matrix comprising at least 40 weight % of a metal selected from the group consisting of iron, cobalt and nickel and from 10 to 40 weight % of aluminum, silicon and chromium. The balance of the matrix is one or more conventional metals used to impart mechanical strength and/or corrosion resistance. The oxides may be simple or complex such as spinels. The patent teaches that the oxides should not be present in the matrix in a volume fraction greater than about 50%, otherwise the surface has insufficient ductility, impact resistance and resistance to thermal fatigue. The outermost surface of the present invention covers at least 55% of the stainless steel (e.g. at least 55% of the outer or outermost surface of the stainless steel has the composition of the present invention).
U.S. Pat. No. 5,536,338 issued Jul. 16, 1996 to Metivier et al. (assigned to Ascometal S.A.) teaches annealing carbon steels rich in chromium and manganese in an oxygen rich environment. The treatment results in a surface scale layer of iron oxides slightly enriched in chromium. This layer can easily be removed by pickling. Interestingly, there is a third sub-scale layer produced which is composed of spinels of Fe, Cr and Mn. This is opposite to the subject matter of the present patent application.
U.S. Pat. No. 4,078,949 issued Mar. 14, 1978 to Boggs et al. (assigned to U.S. Steel) is similar to U.S. Pat. No. 5,536,338 in that the final surface sought to be produced is an iron based spinel. This surface is easily subject to pickling and removing of slivers, scabs and other surface defects. Again this art teaches away from the subject matter of the present invention.
U.S. Pat. No. 5,630,887 issued May 20, 1997 to Benum et al. (assigned to Novacor Chemicals Ltd. (now NOVA Chemicals Corporation)) teaches the treatment of stainless steel to produce a surface layer having a total thickness from about 20 to 45 microns, comprising from 15 to 25 weight % of manganese and from about 60 to 75 weight % of chromium. Clearly the patent requires the presence of both manganese and chromium in the surface layer but does not teach a spinel. The present invention requires a surface predominantly of a spinel of the formula MnxCr3-xO4 wherein x is from 0.5 to 2. The reference fails to teach the surface composition of the present invention.
The present invention seeks to provide a surface having extreme inertness (relative to coke make) and sufficient thermo-mechanical stability to be useful in commercial applications. The present invention also seeks to provide an outermost surface on steels which surface provides enhanced materials protection (e.g. protects the substrate or matrix).